Logarithmic, Exponential, and Other Transcendental Functions Copyright © Cengage Learning. All rights reserved.

The Natural Logarithmic Function: Differentiation Copyright © Cengage Learning. All rights reserved.

Objectives Develop and use properties of the natural logarithmic function. Understand the definition of the number e. Find derivatives of functions involving the natural logarithmic function.

The Natural Logarithmic Function

The Natural Logarithmic Function The General Power Rule has an important disclaimer—it doesn’t apply when n = –1. Consequently, you have not yet found an antiderivative for the function f(x) = 1/x. In this section, you will use the Second Fundamental Theorem of Calculus to define such a function. This antiderivative is a function that you have not encountered previously in the text.

The Natural Logarithmic Function It is neither algebraic nor trigonometric, but falls into a new class of functions called logarithmic functions. This particular function is the natural logarithmic function.

The Natural Logarithmic Function

The Natural Logarithmic Function From this definition, you can see that ln x is positive for x > 1 and negative for 0 < x < 1, as shown in Figure 5.1. Moreover, ln(1) = 0, because the upper and lower limits of integration are equal when x = 1. Figure 5.1

The Natural Logarithmic Function To sketch the graph of y = ln x, you can think of the natural logarithmic function as an antiderivative given by the differential equation Figure 5.2 is a computer-generated graph, called a slope (or direction) field, showing small line segments of slope 1/x. The graph of y = ln x is the solution that passes through the point (1, 0). Figure 5.2

The Natural Logarithmic Function The next theorem lists some basic properties of the natural logarithmic function.

The Natural Logarithmic Function Using the definition of the natural logarithmic function, you can prove several important properties involving operations with natural logarithms. If you are already familiar with logarithms, you will recognize that these properties are characteristic of all logarithms.

Example 1 – Expanding Logarithmic Expressions

Example 1 – Expanding Logarithmic Expressions cont’d

The Natural Logarithmic Function When using the properties of logarithms to rewrite logarithmic functions, you must check to see whether the domain of the rewritten function is the same as the domain of the original. For instance, the domain of f(x) = ln x2 is all real numbers except x = 0, and the domain of g(x) = 2 ln x is all positive real numbers. (See Figure 5.4.) Figure 5.4

The Number e

The Number e It is likely that you have studied logarithms in an algebra course. There, without the benefit of calculus, logarithms would have been defined in terms of a base number. For example, common logarithms have a base of 10 and therefore log1010 = 1. The base for the natural logarithm is defined using the fact that the natural logarithmic function is continuous, is one-to-one, and has a range of ( , ).

The Number e So, there must be a unique real number x such that lnx = 1, as shown in Figure 5.5. This number is denoted by the letter e. It can be shown that e is irrational and has the following decimal approximation. Figure 5.5

The Number e Once you know that ln e = 1, you can use logarithmic properties to evaluate the natural logarithms of several other numbers.

The Number e For example, by using the property ln(en) = n ln e = n(1) you can evaluate ln (en) for various values of n as shown in the table and in Figure 5.6. Figure 5.6

The Number e The logarithms shown in the table are convenient because the x–values are integer powers of e. Most logarithmic functions are, however, best evaluated with a calculator.

Example 2 – Evaluating Natural Logarithmic Expressions

The Derivative of the Natural Logarithmic Function

The Derivative of the Natural Logarithmic Function The derivative of the natural logarithmic function is given in Theorem 5.3. The first part of the theorem follows from the definition of the natural logarithmic function as an antiderivative. The second part of the theorem is simply the Chain Rule version of the first part.

The Derivative of the Natural Logarithmic Function

Example 3 – Differentiation of Logarithmic Functions

The Derivative of the Natural Logarithmic Function On occasion, ot is convenient to use logarithms as aids in differentiating nonlogarithmic functions. This procedure is called logarithmic differentiation.

Example 6 – Logarithmic Differentiation Find the derivative of Solution: Note that y > 0 for all x ≠ 2. So, ln y is defined. Begin by taking the natural logarithm of each side of the equation. Then apply logarithmic properties and differentiate implicitly. Finally, solve for y'.

Example 6 – Solution cont’d

The Derivative of the Natural Logarithmic Function Because the natural logarithm is undefined for negative numbers, you will often encounter expressions of the form The next theorem states that you can differentiate functions of the form as though the absolute value notation was not present.

Example 7 – Derivative Involving Absolute Value Find the derivative of f(x) = ln | cosx |. Solution: Using Theorem 5.4, let u = cos x and write